6G7 Bandmaster style tone stack questions???

[jimboyogi]

Hi All,

I have been playing around with this style of tone stack. The original is below -



This stack is also in Merlin B's pre-amp books, but there has been impedance scaled by doubling the resistances, and modifying the capacitances.
In the first edition of the book, the 10nF cap was /2 to 4.7nF, but the treble cap was doubled from 250pF to 500pF.
In the second edition, the 10nF cap was left at 10nF, with a 500pF treble cap.

In the text of the book it outlines how decreasing the treble cap value raises the corner freq. of the treble filter, thereby deepening the mid scoop, as well as (I am assuming) increasing the scoop centre freq.

But I am so far unable to understand how to modify the freq. responses in other ways, e.g. to raise the corner freq. of the bass filter, response changes when driven from lower impedances, or to make the scoop centre freq. adjustable etc.

Can anyone please help me understand how the circuit works, so that I can then modify the response to suit my needs?
Or does anyone know a simulator that works on this tone stack?

Cheers!

[jazbo8]

You can use the Duncan TSC to see how the values interact. Select "Vox" type.

[Alan0354]

Are you going to mod the amp? One way is to try different value caps. I am not sure looking at the graph can tell a whole lot.

[jimboyogi]

Thanks for the replies!

jazbo8, unfortunately I use a Mac, and I have so far been unable to load and run the Duncan TSC. I am currently trying to fire up my old PC to see if I can run it there.

Alan0354, I wish that I had a 6G7! If I did I doubt that I would mod it, just play and enjoy it
I have used this Bandmaster tone stack in a couple of builds so far, and I have played around with some of the values to see what tonal difference they make. But this has been very empirical, and I was hoping to understand the circuit better so that I could predict the effects of component changes, then fine tune by ear.

[jimboyogi]

OK so I have got the duncan TSC working on my old PC. Thanks for the tip about equating this Bandmaster stack with the Vox one jazbo8!

The only difference that I can see between the 2 circuits is the 1 extra cap (labelled C2) that runs from between the treble and bass pot legs across, where in the bandmaster circuit this is a wire link. I have just made this cap 100uF, am I right in thinking that this will equate to a wire link for AC?

[jazbo8]

Yes, pretty close, good enough for rock 'n roll

[eschertron]

OK so I have got the duncan TSC working on my old PC. Thanks for the tip about equating this Bandmaster stack with the Vox one jazbo8!

The only difference that I can see between the 2 circuits is the 1 extra cap (labelled C2) that runs from between the treble and bass pot legs across, where in the bandmaster circuit this is a wire link. I have just made this cap 100uF, am I right in thinking that this will equate to a wire link for AC?

If you type in 1M (capital 'm') then you'll simulate that wire with a 1MegaFarad cap. If it makes a difference!

[jimboyogi]

That tone stack calculator is amazing! Hours of fun plugging different values in, and seeing what happens

Next question - how accurate is it to reality?

Also WRT the cap in place of wire link, anything above 10uF makes no further difference to the response chart. I haven't gone any lower to see at what capacitance it begins to change.

[jazbo8]

The result should roughly corresponds to what you hear, but just remember that the single biggest variable - the speaker is unaccounted for...